Many human tumor cells are chromosomally unstable, showing an elevated rate of gains and losses of whole chromosomes that is 10-100 times higher than that seen in diploid primary cells. Chromosome instability (CIN) enhances the evolution of tumor cells causing genomic changes that can promote metastasis and chemotherapeutic resistance. CIN can have a causal role in tumorigenesis and it correlates with poor patient prognosis. Identifying the mutations that cause CIN in human cancer, and understanding why they reduce the fidelity of mitosis, are important objectives in cancer research. The inactivation of the pRB pathway promotes cell proliferation and is a common event in tumor cells. Interestingly, the functional inactivation of pRB causes aneuploidy. Recent experiments show that the specific loss of pRB increases rates of chromosome mis-segregation to levels that are remarkably similar to CIN tumor cells. This strongly suggests that a significant fraction of the chromosome instability seen in tumor cells is a byproduct of the inactivation of pRB. This grant investigates the exciting link between pRB inactivation and chromosome mis-segregation. Newly obtained results show that the loss of pRB causes defects in centromere function and chromatid cohesion. We will test the hypothesis that defects in cohesion and condensation allow chromosome mis-segregation when pRB-deficient cells are delayed in mitosis. Cohesin is the primary determinant of chromosome cohesion. In Aim 1 we will determine how the association of cohesin complexes with chromatin is altered in the absence pRB, and will test the functional significance of physical interactions between pRB and cohesin. The experiments in Aim 2 will test whether the mitotic defects in pRB-deficient cells can be either suppressed or enhanced. Such experiments may lead to new therapeutic opportunities. In Aim 3 we will extend these studies to cancer cells and will measure and compare the changes in centromere function, chromosome cohesion, and the rates of chromosome mis-segregation seen when functional pRB is re-introduced into tumor cells that lack it, or is specifically removed from tumor cells with functional pRB. We will determine whether similar changes occur when pRB is inactivated by deregulated cdk activity and when pRB-related proteins are targeted.